The fuel used in nuclear power stations changes over time as it is used. It is commonplace for this change to be analysed and, in order to do so, fuel elements are removed then evaluated in laboratories equipped with enclosures suited to reducing exposure to radiation, for example using thick lead walls. These enclosures are referred to as high-activity cells or hot cells.
The fuel analysed usually takes the form of an irradiated fuel rod. Tools and methods have been developed over the years for studying these.
The evaluations carried out in the hot cells may be destructive, physical intervention on the rod then being required. By way of example, the rod may therefore be cut into sections for this type of evaluation.
Alternatively, evaluations known as non-destructive evaluations and denoted by the abbreviation NDE may also be carried out. In that case, the geometry of the rod remains unaltered. Non-destructive evaluations can be carried out by measuring the radiation emitted by the isotopes resulting from the fission of a fuel such as uranium-235, denoted U-235 throughout the rest of the description. By way of example, gamma-ray spectrometry, the principle of which is to identify radioactive elements by measuring the energy of the gamma radiation (photons) emitted, provides damage-free access to information directly connected with the fuel contained in the rod.
With knowledge of the gamma spectral lines of the isotopes that are to be studied it is possible to take measurements per region of interest in order to obtain a surface in measurement steps and by isotope. The distribution of the fission products created by the irradiation and visible in gamma-ray spectrometry provides direct access to the features of the fuel stack, such as the start of the stack and the end of the stack and provides information regarding the irradiation. In this description, the expression “fuel stack” denotes a collection of at least two pellets which are aligned and placed one behind the other. A rod is an assembly made up of a fuel stack, of a cladding comprising the said stack and of two end plugs, one at each end of the cladding.
Rods for pressurised water reactors, referred to as PWR rods, are made up of a cylindrical cladding containing a plurality of fuel pellets positioned one after another longitudinally inside the said cladding. The pellets are of cylindrical shape and have a length of the order of one centimetre. At their ends they have chamfers and cavities (usually denoted by the English word “dishing”), one of the purposes of this being to allow them to absorb deformations.
The special geometry of the pellets leads to reductions in the material at their ends. The result of this is that the gamma count rates for the fission products around these zones are lower. This variation in the count rate thus makes it possible to distinguish the pellets from one another.
To complement this, these qualitative spectrometry measurements can be supplemented by quantitative analyses. Thus, information such as the number of caesium-137 (Cs-137) atoms per pellet makes it possible to estimate the rate of combustion per pellet.
Multi-channel analysers are usually employed for analysing the gamma spectrum. These analysers have as output data histograms made up of several channels. One histogram channel j corresponds for example to one energy band Bj. The quantities to be measured pertaining to the gamma photons are classified by channels according to their energy level. The set of channels j indicative of a spectrum make up a histogram.
The quantity measured for a channel j corresponds to a number of events measured in a given time, also known as counts. For each channel, the counts measured are counted up, making it possible to determine the value associated with the said channels.
The counts correspond to measurements indicative of interactions between the particles that are to be measured and the sensor used, for example the germanium crystal. In other words, when a particle is detected by the sensor, a count is counted. The histogram is usually represented with the energy level along the abscissa axis and the number of counts on the ordinate axis (in which case it gives a “count”) or as a number of counts per second (which then gives a count rate).
Non-destructive NDE evaluations of irradiated fuel rods using gamma-ray spectrometry notably make it possible to locate the zones separating two adjacent pellets, these zones being referred to in the remainder of the description as inter-pellet zones. However, there is a need to know the histories behind the temperatures experienced by the fuel pellets during their cycle in the reactor in order to connect the post-irradiation results to these histories and further the understanding of the phenomena that occur in a reactor. The objective of gaining this knowledge is notably to improve the methods of manufacture of these fuels, pellets and rods, to better control the reactor or even manage accidents.
Now, these evaluation methods do not allow the effect the temperature has on the rods to be analysed specifically, i.e. does not allow the zones of the rod that have been heated up to be both identified and quantified.
One of the objects of the invention is to address these insufficiencies of the prior art and to make improvements thereto.